where \(\mathcal {F}\)  is the gradient deformation tensor which is contracted with the viscous tensor \(\tau ^\nu \) , \(\nabla  N_i\)  is the gradient of the shape functions and \(A_m\)  is the surface area of the \(m\) -th face of the triangular mesh [1]}, [2]}.
More details are given in [3]}, [4]}.
